The Czochralski method is a representative method for the growth of a single crystal from a melt.
The Czochralski method uses a crucible 2 provided in a closed chamber 1 as shown in FIG. 1. The crucible 2 is supported by a support 3 so that tile crucible 2 is capable of rotation and/or vertical motion. There are concentrically disposed a heater 4 and a heat insulator 5 around the crucible 2.
A raw material is received in the crucible 2 and intensively heated by the heater 4 to prepare a melt 6. The melt 6 is held at a temperature suitable for the growth of a single crystal.
When a seed crystal 7 being hung from a rotary winder 10 through a wire 9 is brought into contact with the melt 6, a single crystal 8 grows on the seed crystal 7 so that the crystalline orientation of the seed crystal 7 is transferred to that of the growing crystal 8. The seed crystal 7 is then rotatingly pulled up in response to the growth of the single crystal 8. The crucible 2 is descendingly rotated by the rotating motion of the support 3, too. The descending speed and rotating speed of the support 3, the ascending speed and rotating speed of the seed crystal 7, etc. are controlled in response to the growing speed of the single crystal 8 being pulled up from the melt 6.
When a melt 6 mixed with Sb as a type-n impurity is used in the pull method, Sb is introduced into an obtained single crystal 8. Hereby, a semiconductor material having high conductivity is obtained.
In addition, the melt 6 contains oxygen originated in SiO.sub.2 dissolved from the crucible. Said oxygen is intorduced into the single crystal 8, too. Oxygen included in the single crytal 8 is precipitated in bulk during the heat treatment of the single crystal 8, resulting in the formation of precipitation faults. The faults serve as a gettering center for capturing heavy metal impurities, which remain on the surface layer of a semiconductor single crystal substrate to be incorporated in an electronic device, and render the impurities into a harmless state. Oxygen dissolved in the single crystal effectively improves the strength of the semiconductor single crystal substrate.
In this consequence, it is preferable to maintain the oxygen concentration of the melt at a higher level, so that the higher oxygen concentration of the melt increases the concentration of oxygen to be incorporated in the single crystal. However, it is difficult to maintain the oxygen concentration of the melt at a higher lever under stable conditions.
We have found the effect of Sb on oxygen concentration during the research and investigation of the physical property of a Si melt. When a large amount of Sb is added to the Si melt, the oxygen concentration of the Si melt becomes higher linearly with the increase of Sb content in the Si melt. The relationship of Sb content with oxygen concentration can be utilized for calculating the oxygen concentration of the Si melt from the Sb content, as we have disclosed in Japanese Patent Application 5-69924.
A Si melt containing a large amount of Sb exhibits the tendency to accelerate the diffusion of oxygen as Sb.sub.2 O, SiO, etc. from the surface of the melt into the atmosphere. This tendency is recognized in another Si melt doped with different Group-V element such as P, As or Bi.
As oxygen is diffused from the surface of the melt, the oxygen concentration of the melt changes so that the oxygen concentration of an obtained Si single crystal is remarkably reduced. Hereby, the concentrations of Sb and oxygen becomes lower due to the evaporation of Sb.sub.2 O, during a melt homogenizing period from the addition of Sb to the start of pulling operation. If the concentration becomes unexpectedly lower, it is necessary to re-prepare another melt. As a result, the productivity becomes worse, and the obtained single crystal has unstable oxygen concentration.